Theoretical investigations of half-metallic ferromagnetism in new Half-Heusler YCrSb and YMnSb alloys using first-principle calculations

Structural,electronic,and magnetic properties of new predicted half-Heusler YCrSb and YMnSb compounds within the ordered MgAgAs Clb-type structure are investigated by employing first-principal calculations based on density functional theory.Through the calculated total energies of three possible atomic placements,we find the most stable structures regarding YCrSb and YMnSb materials,where Y,Cr(Mn),and Sb atoms occupy the(0.5,0.5,0.5),(0.25,0.25,0.25),and(0,0,0) positions,respectively.Furthermore,structural properties are explored for the non-magnetic and ferromagnetic and anti-ferromagnetic states and it is found that both materials prefer ferromagnetic states.The electronic band structure shows that YCrSb has a direct band gap of 0.78 eV while YMnSb has an indirect band gap of 0.40 eV in the majority spin channel.Our findings show that YCrSb and YMnSb materials exhibit half-metallic characteristics at their optimized lattice constants of 6.67  and 6.56 ,respectively.The half-metallicities associated with YCrSb and YMnSb are found to be robust under large in-plane strains which make them potential contenders for spintronic applications.     

Co doping effects on structural, electronic and magnetic properties in Mn2VGa

The electronic structure and magnetic properties of Co-doped Heusler alloys (Mn_1−xCo_x)₂ VGa (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied by first-principles calculations. The results show that the lattice constants decrease with increasing Co content except x=1.0. The spin polarization for x=0.5 is only 34%, much lower than the other concentrations. The compounds of x=0.0, 0.25 show nearly half-metallicity because the Fermi level slightly touches the valence bands. And the compounds of x=0.75, 1.0 exhibit the half-metallic character with 100% spin polarization. It is found the local moments of Mn(Co) basically show a linear increasing trend while the moments of V show a linear decreasing trend with increasing doping concentration. However, the local moments for x=0.5 quite depart from the linear trend. The majority-spin component at the Fermi level increases while the minority-spin component at the Fermi level decreases with the substitution of Co atoms for Mn atoms when x≤0.75. For x≥0.75, the majority-spin component remains more or less the same and the gap in the minority DOS increases with Co doping. The majority spin states are shifted to valence bands and the majority spin states around E_F increase due to a leakage of charge from the unoccupied spin-up states to the occupied majority states with increasing Co content.     

FeMnP_(1-x)T_x(T=Si,Ga,Ge)系列化合物机械性能的第一性原理研究

以密度泛函理论为基础,使用投影缀加波方法、VASP程序包研究了FeMnP_(1-x)T_x(T=Si,Ga,Ge)化合物的力学性质,结果表明FeMnP_(1-x)Ga_x化合物的晶格参数、弹性常数和电子结构与FeMnP_(1-x)Ge_x化合物比较接近,同时该化合物在力学上稳定,是预期具有较大的磁熵变和高磁热效应的材料.依据Pugh判据,FeMnP_(0.67)T_(0.33)(T=Si,Ga,Ge)化合物具有良好的延展性,三者之中FeMnP_(0.67)Ga_(0.33)韧性最好,FeMnP_(0.67)Si_(0.33)韧性相对较差,说明Ga替代P可改善此类化合物的机械性能.最后从化合物体系电子总态密度随不同掺杂T原子的演化规律解释了自洽计算得到的弹性常数的变化规律.     

Co_(2~-)基Heusler合金Co_2FeAl_(1–x)Si_x(x=0.25,x=0.5,x=0.75)的结构、电子结构及热电特性的第一性原理研究

运用基于密度泛函理论的第一性原理方法,对Co_2FeAl_(1–x)Si_x(x=0.25, 0.5, 0.75)系列Heusler合金的电子结构、四方畸变、弹性常数,声子谱以及热电特性进行了计算研究.结果显示, Co_2FeAl_(1–x)Si_x系列合金的电子结构均为半金属特性,向下自旋态(半导体性)均呈现良好的热电特性,并且随着硅原子浓度的增加功率因子随之增加.计算的声子谱不存在虚频,均满足动力学稳定性条件,弹性常数均满足玻恩稳定性条件,机械稳定性均良好.随着晶格常数c/a的比值变化,体系的能量最低点均出现在c/a=1处,即结构稳定性不随畸变度c/a的变化而变化,说明不存在马氏体相变.此系列合金薄膜的电子结构呈现较高的自旋极化率,在替代浓度x=0.75时自旋极化率达到100%,且当x=0.75时薄膜在畸变度c/a=1.2时存在马氏体相变.随着晶格畸变度的改变,总磁矩也发生变化,且主要由Fe和Co两种过渡金属原子的磁矩变化所决定.     

First-principles study of electronic structure and magnetic properties of SrTi1−xMxO3 (M= Cr,Mn, Fe,Co, or Ni)

We used first-principles calculations to conduct a comparative study of the structure and the electronic and magnetic properties of SrTiO₃ doped with a transition metal (TM), namely, Cr, Mn, Fe, Co, or Ni. The calculated formation energies indicate that compared with Sr, Ti can be substituted more easily by the TM ions. The band structures show that SrTi_0.875Cr_0.125O₃ and SrTi_0.875Co_0.125O₃ are half metals, SrTi_0.875Fe_0.125O₃ is a metal, and SrTi_0.875Mn_0.125O₃ is a semiconductor. The 3d TM-doped SrTiO₃ exhibits various magnetic properties, ranging from ferromagnetism (Cr-, Fe-, and Co-doped SrTiO₃) to antiferromagnetism (Mn-doped SrTiO₃) and nonmagnetism (Ni-doped SrTiO₃). The total magnetic moments are 4.0_μB, 6.23_μB, and 2.0_μB for SrTi_0.75Cr_0.25O₃, SrTi_0.75Fe_0.25O₃, and SrTi_0.75Co_0.25O₃, respectively. Room-temperature ferromagnetism can be expected in Cr-, Fe-, and Co-doped SrTiO₃, which agrees with the experimental observations. The electronic structure calculations show that the spin polarizations of the 3d states of the TM atoms are responsible for the ferromagnetism in these compounds. The magnetism of TM-doped SrTiO₃ is explained by the hybridization between the TM-3d states and the O-2p states.     

Magnetic properties of (Ga,Mn)N ternaries and structural,electronic, and magnetic properties of cation-mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries

ABINIT simulation package with built in local density, generalized gradient, and spin local density approximations was used to investigate the structural, electronic, and magnetic properties of cation mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal and fixed compositions of Ga, In, and Mn atoms. In particular, total energy minimization approach was used to compute the equilibrium structural parameters of zinc-blende (GaAs, InAs, and MnAs), wurtzite (GaN, InN, and MnN) binary parent compounds, as well as, the corresponding equilibrium parameters of (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternary systems. The band structures of zinc-blende GaAs, InAs, and MnAs binary parent compounds were computed and analyzed. Spin polarized band structures of the cation mixed (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal compositions of Ga, In, and Mn cations were computed and analyzed using spin local density approximation based calculations. Moreover, the magnetic properties of (Ga,Mn)(As,N) and (In,Mn)(As,N) quaternaries with equal concentration of Ga, In, and Mn cations were investigated. Our results suggest that the two quaternary systems are nonmagnetic. An interpretation of our results is presented. In addition, the magnetic properties of (Ga,Mn)N nanocrystal ternaries constructed from doping GaN with one or two Mn atoms were investigated using Vienna Ab-initio Simulation Package (VASP) and compared with those of (Ga,Mn)(As,N) quaternaries.     

Magnetic and Magneto-optical properties of Heusler alloys of the type Ni3-xMnxSn

The crystallographic, magnetic and magneto-optical properties of Heusler compounds of the type Ni_3-xMn_xSn were investigated. The thermal expansion behaviour of a limited number of these compounds was also studied. The cubic L2₁ structure was found to exist in the range 0.6 x 2.2. With increasing x the saturation moment first increases, then decreases again. The main peak in the Kerr rotation spectra measured in the energy range 0.5-4.5 eV shows a similar trend. The Curie temperature and the lattice constant increase with x, both quantities showing a discontinuity near x = 1. All these features are discussed in terms of relative site occupancies of the Mn atoms at the tetrahedral and octahedral sites in the L2₁ structure.     

Magnetic linear dichroism in the angular dependence of core-level photoemission from (Ga,Mn)As using hard x rays

We report a study of the electronic properties of the ferromagnetic semiconductor (Ga,Mn)As using magnetic linear dichroism in the angular dependence of Mn 2p photoemission under hard x-ray excitation. Bulk plasmon loss satellites demonstrate that the probed Mn ions are incorporated deep within the GaAs lattice, while the observed large dichroism indicates that the spectra originate from ferromagnetic substitutional Mn. Simulations of the spectra using an Anderson impurity model show that the ferromagnetic Mn 3d electrons of substitutional Mn in (Ga,Mn)As are intermediate between localized and delocalized.     

(Ga,Mn)As 体系中Mn自补偿效应的第一性原理研究

基于密度泛函理论的赝势平面波方法计算了处于填隙位置磁性原子(MnI)对(Ga,Mn) As体系电子结构和磁性的影响. 计算结果表明MnI在GaAs中是施主；代替Ga位的MnGa 与MnI的自旋按反铁磁序排列；静电相互作用使MnGa，MnI倾向于形成MnGa_MnI对. MnI的存在一方面补偿了(Ga,Mn)As中的空穴，降低了空穴浓度；同时还使邻近的MnG a失活. MnI的存在对获得高居里温度的(Ga,Mn)As是极为不利的. 关键词： (Ga Mn)As 稀磁半导体 密度泛函理论     

A structural model of the orthorhombic Ga7Mn5 derived from its HREM image

The base-centered orthorhombic approximant Ga7Mn5 (a = 4.547(6) nm, b = 1.256(9) nm, c = 1.482(1) nm) consisting of flattened hexagon and concave octagon (called "crown" in this paper) subunits shown by high-resolution electron microscopy (HREM) was found to coexist with the Ga-Mn decagonal quasicrystal in the Ga58Mn42 alloy. The atomic sites in the flattened hexagon subunit have been deduced recently from a structural study of the orthorhombic Ga-Mn (1/1,1/1) approximant that consists only of the hexagon subunits. Similarly, the atomic sites in the crown subunit have been deduced from another orthorhombic Ga-Mn (2/1,1/1) approximant consisting only of the crown subunits. A structural model of the orthorhombic Ga7Mn5 can thus be constructed from these structural subunits. There are 332 Ga atoms and 232 Mn atoms in a unit cell, with a composition of Mn41.2Ga58.8, satisfying the symmetry of the space group Bmm2. The simulated electron diffraction patterns, [010] HREM images, and X-ray powder diffraction pattern based on this structural model agrees mainly with the experimental observations.     

Anisotropic exchange interaction in CdTe/Cd0.75Mn0.25Te quantum wires

We observed anisotropic large peak-energy shift of photoluminescence (PL) from CdTe/Cd_0.75Mn_0.25Te quantum wires (QWRs). The large PL-peak-energy shift is caused by the exchange interaction between excitons in the non-magnetic QWR and Mn ions in the DMS barrier. The exchange interaction is enhanced when magnetic field is perpendicular to the QWR.     

The origin and control of the sources of AMR in (Ga,Mn)As devices

We present details of our experimental and theoretical study of the components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As. We develop experimental methods to yield directly the non-crystalline and crystalline AMR components which are then independently analysed. These methods are used to explore the unusual phenomenology of the AMR in ultra thin (5 nm) (Ga,Mn)As layers and to demonstrate how the components of the AMR can be engineered through lithography induced local lattice relaxations. We expand on our previous [A.W. Rushforth, et al., Phys. Rev. Lett. 99 (2007) 147207] theoretical analysis and numerical calculations to present a simplified analytical model for the origin of the non-crystalline AMR. We find that the sign of the non-crystalline AMR is determined by the form of the spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential.     

Quaternary Heusler compounds Co(2-x)Rh(x)MnZ (Z = Ga, Sn, Sb): crystal structure, electronic structure, and magnetic properties

Within the huge family of Heusler compounds only a few quaternary derivatives are known that crystallize in the F43m space group. In this work, the yet unreported compounds CoRhMnZ (Z = Ga, Sn, Sb) and the alloy Co(0.5)Rh(1.5)MnSb were investigated in detail by experimental techniques and theoretical methods. The ab initio calculations predict the CoRhMnZ compounds to be half-metallic ferromagnets or to be close to the half-metallic ferromagnetic state. Calculations of the elastic constants show that the cubic structure is stable in compounds containing Mn. Both calculations and experiment reveal that Mn cannot be exchanged by Fe (CoRhFeGa). The low temperature magnetization of the compounds is in the range of 3.4-5.5 μ(B) depending on the composition. The best agreement between experiment and calculation has been achieved for CoRhMnSn (5 μ(B)). The other compounds are also cubic but tend to anti-site disorder. Compared to Co(2)MnSn it is interesting to note that the magnetic properties and half-metallicity are preserved when replacing one of the 'magnetic' Co atoms by a 'non-magnetic' Rh atom. This allows us to increase the spin-orbit interaction at one of the lattice sites while keeping the properties as a precondition for applications and physical effects relying on a large spin-orbit interaction. The Curie temperatures were determined from measurements in induction fields of up to 1 T by applying molecular field fits respecting the applied field. The highest Curie temperature was found for CoRhMnSn (620 K) that makes it, together with the other well defined properties, attractive for above room temperature spintronic applications.     

Doping effect on the structure and physical properties of quasi-one-dimensional compounds Ba_9Co_3(Se_(1-x)S_x)15(x=0–0.2)

A series of samples of Ba_9Co_3(Se_(1-x)S_x)15 (x=0,0.05,0.1,0.15,0.2) with quasi-one-dimensional (1D) structure were successfully synthesized under high-temperature and high-pressure conditions.The influence of partial substitution of S for Se on the structure,electronic transport,and magnetic properties of Ba_9Co_3(Se_(1-x)S_x)15 has been investigated in detail.The x-ray diffraction data shows that the lattice constant decreases linearly with increasing S-doping level,which follows the Vegrad’s law.The doped S atoms preferentially occupy the site of Se atoms in CoSe_6 octahedron.Physical properties measurements indicate that all the samples of Ba_9Co_3(Se_(1-x)S_x)15 are semiconducting and display spin glass behavior.As the replacement of Se by smaller size S,although the inter-chain distance decreases,the electronic hopping between CoSe/S_6 chains is weakened and leads to an increase of band gap from 0.75 e V to 0.86 e V,since the S-3p electrons are more localized than Se-4p ones.Ba_9Co_3(Se_(1-x)S_x)15 exhibits 1D conducting chain characteristic.     

Chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape-memory alloys studied with neutron diffraction

The chemical order of three off-stoichiometry Ni–Mn–Ga compositions has been measured in the austenitic phase using powder and single-crystal neutron diffraction. The compositions studied, 48–52 at.% nickel, having excess manganese and deficient in gallium, are of technical interest due to the observed large room-temperature, magnetic-field-induced strain. It has been determined that compositions with less than 50% nickel have the excess Mn atoms occupying Ni and Ga sites. Compositions enriched in nickel are best fit with Ni atoms in excess of 50% occupying Mn sites while the excess and displaced Mn occupy Ga sites. The saturation magnetic moments calculated from the site occupations determined here and using Ni and Mn moments reported for Ni₂MnGa, agree within 4% with the low-temperature measured moments.     

Magnetism in artificial lattices

We compare magnetism in two artificial lattice structures, a quantum dot array formed in a two-dimensional electron gas and an optical lattice loaded with repulsive, contact-interacting fermionic atoms. When the tunneling between the lattice sites is strong, both lattices are non-magnetic. With reduced tunneling in the tight-binding limit, the shell-filling of the single-site quantum wells combined with Hund's rule determines the magnetism. This leads to a systematic magnetic phase diagram with non-magnetic, ferromagnetic and antiferromagnetic phases.     

Pr含量对Bi_5Fe_(0.5)Co_(0.5)Ti_3O_(15)室温多铁性的影响

采用改良的固相烧结工艺制备了Bi5-xPrxFe0.5Co0.5Ti3O15(BPFCT-x,x=0.25,0.50,0.75,0.80)陶瓷样品.X射线衍射结构分析表明:镨(Pr)含量对样品微观结构产生了影响,但所有样品均为层状钙钛矿结构;BPFCT-x样品的剩余极化强度(2Pr)随着掺杂量的增加呈现出先增大后减小的变化趋势,当Pr含量为0.75时,样品的2Pr达到最大值,为6.43μC/cm2.样品的磁性与铁电性能具有相同的变化规律,室温下样品的剩余磁化强度(2Mr)也呈现出先增大后减小的趋势,并且也在x=0.75时达到最大为0.097 emu/g.随着Pr掺杂量增大,样品的室温下铁电和铁磁性能得到明显改善,并且当掺杂量为0.75时,样品室温多铁性最好.Pr掺杂降低了样品中的缺陷浓度,从而提高了样品铁电畴动性,这有助于提高样品铁电性能.而样品铁磁性能的改善可能与Pr对样品晶格畸变产生的影响有关.     

3d过渡金属在NiAl中的占位及对键合性质的影响

通过赝势平面波方法系统地研究了3d过渡金属元素在B2-NiAl中的占位以及对键合性质的影响.通过形成能得出Sc,Ti,V 和Zn元素优先取代NiAl中的Al位,而Cr,Mn,Fe,Co和Cu优先取代Ni位.通过分析晶格常数变化量、电荷聚居数、交叠聚居数以及价电荷密度分布, 讨论了晶格畸变和键合性质的变化.结果表明: 取代Al的Sc,Ti,V和Zn元素掺杂使NiAl中晶格发生畸变,这对NiAl键合性质的变化起着重要作用,这些掺杂元素与第一近邻Ni原子产生强烈的排斥作用,形成反键,同时它们之间发生较大的电荷转 关键词： NiAl金属间化合物 3d过渡金属 第一性原理 键合性质